A practical problem has prevented the widespread use and availability of high fidelity hearing aids. Specifically, dampers, which are used to smooth the frequency response, often needed to be near the tip of the hearing aid outlet at a point where they are easily clogged with ear canal wax.
As a result, hearing aid manufacturers stopped using dampers near the eartip, and unpleasant peaks in the frequency response became commonplace. This problem was recognized by Killion et al. in U.S. Pat. No. 5,812,679 issued Sep. 22, 1998 entitled “Electronic Damper Circuit for a Hearing Aid and Method of Using the Same” and in U.S. Pat. No. 6,047,075 issued Apr. 4, 2000 entitled “Damper for Hearing Aid.” These patents describe the use of electronic filtering to substitute for the acoustic damper. One of the realizations at the time was that by making the filter programmable, it could be adjusted to accommodate the different peak frequencies that are obtained when different lengths of tubing are used with the earphone to accommodate different lengths of ear canals and earmolds.
Although the electronic damping of Killion et al. was a substantial contribution, we now have realized additional problems in making a completely high fidelity hearing aid. Even though the response with different receiver “plumbing” arrangements can be adequately damped, the finished frequency response may not produce a full fidelity hearing aid. In other cases, the model of receiver that is chosen on the basis of power handling or other considerations, may have its peak frequency placed well below 2 kHz. In this situation, according to the prior art, a high fidelity response becomes nearly impossible, regardless of the choice of damping or plumbing.
To explain, a full fidelity hearing aid generally must have a frequency response matching one of the “CORFIG” responses described by Killion and Monser (CORFIG: Coupler Response for Flat Insertion Gain by Mead C. Killion and Edward L. Monser, IV, in Acoustical Factors Affecting Hearing Aid Performance, Studebaker, G. A. and Hochberg, I., eds., pgs. 149-168, 1980) (Appendix A) and by Killion and Revit (CORFIG and GIFROC: Real Ear to Coupler and Back by Mead C. Killion and Lawrence Revit in Acoustical Factors Affecting Hearing Aid Performance (2nd Ed.), Studebaker, G. A. and Hochberg, I., eds., pgs. 65-86, 1993). The adequately damped peak may, in a particular case, be at a different frequency than the approximately 2.5 kHz frequency of the open ear. In order to have a full fidelity frequency response, it may be necessary to replicate the response that would normally occur at the eardrum without a hearing aid in place. This response is described in the “CORFIG” curve for the type of hearing aid in question (behind-the-ear, in-the-ear, canal aid or completely-in-the-canal aid) as described in Appendix A and in Killion and Revit (mentioned above).
In addition, the microphone response often rolls off above 3 or 4 kHz, making it desirable to further equalize the microphone. This was recognized by Killion et al. in the early application notes for the “K-AMP” integrated circuit chip (as described in ER-101-28D Data Sheet dated 92/7/2) (Appendix B). Capacitor C2S, as described in Note 2 of Appendix B, provided a high frequency boost to compensate for the loss of high frequency response in typical microphones, just as capacitor CHFB produced a high frequency boost to compensate for the loss of high frequency output in typical receivers (Appendix B). A problem arises because microphones must sometimes be mounted at a distance behind the faceplate of the hearing aid and connected to the opening in the faceplate with a section of tubing. Different hearing aids in the same nominal family of hearing aids, therefore, may require different amounts of high frequency correction for the microphone and/or receiver.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.